In semiconductor integrated circuits, a current source provides a reference current in order to design transistor bias networks that are insensitive to supply voltage, temperature, and process variations. If the current through a current source can be specified independently of any other variable in a circuit, it is called an independent current source. Conversely, if the current through a current source is determined by some other voltage in a circuit, it is called a voltage-controlled current source (VCCS).
In most applications, it is desirable to have a linear relationship between the current and the control voltage. However, stand-alone conventional devices do not function as linear VCCSs. For example, in a bipolar junction transistor (BJT) device, the collector current is an exponential function of the base voltage, while in a field-effect transistor (FET) device, the drain current is a power function of the gate voltage. In both cases, the current is a strong function of the control voltage, which makes it unsuitable for linear applications.
Conventionally, complicated circuits using operational amplifiers are required to make a linear VCCS. Such circuits consume extra direct-current (DC) power and are typically large in size.